The present invention relates to a solid electrolytic capacitor utilizing valve metal such as tantalum, niobium or aluminum, for example. The invention particularly relates to a capacitor element used for such a capacitor, a method of making such a capacitor element, and a solid electrolytic capacitor using such a capacitor element.
Conventionally, a capacitor element for use in the above kind of solid electrolytic capacitor is manufactured by such a method as disclosed in JP-A 7-66079, for example. This method comprises the following process steps.    (1) As shown in FIG. 1, powder of valve metal such as tantalum is compacted into a porous parallelepiped anode chip body 2 so that an anode wire 3 projects from a first end surface 2a of the porous chip body 2. Then, the anode chip body is sintered.    (2) Subsequently, the anode chip body 2 is subjected to anodizing. Specifically, direct current is applied, with the anode chip body immersed in a chemical solution such as an aqueous solution of phosphoric acid. As a result, a dielectric film of e.g. tantalum pentoxide is formed on each metal particle of the anode chip body.    (3) Subsequently, as shown in FIG. 1, the anode chip body 2, with the anode wire 3 oriented upward, is immersed into an electrolyte solution A such as an aqueous solution of manganese nitrate so that the electrolyte solution A infiltrates into the porous structure of the anode chip body 2. Thereafter, the anode chip body 2 is pulled out from the electrolyte solution A, and dried and baked. These process steps are repeated a plurality of times. As a result, as shown in FIGS. 2 and 3, a solid electrolyte layer 4 made of metal oxide such as manganese dioxide is formed at the surfaces of the anode chip body 2 via the dielectric film.    (4) Subsequently, a graphite layer is formed at the surfaces of the anode chip body 2 except the end surface 2a.     (5) Then, with the anode wire 3 oriented upward again, the anode chip body 2 is immersed in a metal paste such as silver paste and then pulled out and baked. As a result, as shown in FIG. 4, a cathode electrode film 5 of the metal paste is formed at the surfaces of the anode chip body 2 except the first end surface 2a. 
In this way, a capacitor element 1 is manufactured.
However, in the process step for forming a solid electrolyte layer 4 of metal oxide such as manganese dioxide on the anode chip body 2 via the dielectric film, when the anode chip body 2 is pulled out of the electrolyte solution A, an excess of the electrolyte solution caused to infiltrate into the porous structure of the anode chip body 2 drips from a second end surface 2b, i.e. the lower end surface of the anode chip body 2.
When the excess of the electrolyte solution drips from the second end surface 2b of the anode chip body 2, part of the solution gathers to form droplets, by surface tension, at all of four corner edges 2c′, 2d′, 2e′ and 2f′ of the anode chip body 2 where four side surfaces 2c, 2d, 2e and 2f meet the second end surface 2b. Since the anode chip body is dried and baked in this state, the solid electrolyte layer 4 formed by the above step includes outwardly projecting bumps at all the four corner edges 2c′, 2d′, 2e′ and 2f′ surrounding the second end surface 2b while recessing at the center of the second end surface 2b, as shown in FIGS. 2 and 3.
In this way, the bumps 4′ of the solid electrolyte layer 4 surrounds the second end surface 2b of the anode chip body 2 along the entire periphery of the end surface. Following the above step, after a graphite layer is formed on the anode chip body 2, the anode chip body 2 is immersed in a metal paste such as a silver paste with the anode wire 3 oriented upward and then baked for forming a cathode-side electrode film 5, as shown in FIG. 4. However, when the anode chip body 2 is immersed in a metal paste in the step of forming a cathode-side electrode film, air cannot escape from the recessed portion of the solid electrolyte layer 4 at the second end surface 2b. Therefore, a void is formed due to the trapping of an air bubble 6, which probably causes a defect in the product.
When the cathode-side electrode film 5 is formed to overlap the solid electrolyte layer 4, the cathode-side electrode film 5 also bulges at the four corner edges 2c′, 2d′, 2e′ and 2f′ to form bumps 5′ overlapping the bumps 4′ of the solid electrolyte layer 4.
In this way, the solid electrolyte layer 4 and the cathode-side electrode film 5 are bulged to form bumps 4′ and 5′ at all of the corner edges 2c′, 2d′, 2e′ and 2f′ surrounding the second end surface 2b of the capacitor element 1. The capacitor element 1 is thereafter disposed between an anode lead terminal and a cathode lead terminal so that the anode wire 3 is connected to the anode lead terminal while the cathode-side electrode film 5 is connected to the cathode lead terminal, and the entirety is sealed in a package to provide a complete package-type solid electrolytic capacitor. However, the height and width of the package need be increased by as much as the dimension of the bumps 4′ and 5′ formed at the corner edges 2c′, 2d′, 2e′ and 2f′ surrounding the second end surface 2b of the anode chip body, whereby the size and weight of the capacitor is disadvantageously increased.